1. Field of the Invention
The present invention relates to a fluid analysis apparatus. More particularly, the invention relates to fluid (especially gas) analysis apparatus that incorporates a molecular single electron transistor detector device.
2. Discussion of Prior Art
A variety of techniques are known for the detection and analysis of gas phase analytes. The standard techniques presently used to detect low molecular concentrations typically involve mass spectrometry used in conjunction with pre-concentration techniques and gas chromatography. In recent years, there has also been a drive to produce battery powered portable systems to enable in situ monitoring of, for example, industrial and volcanic emissions. However, such portable devices still weigh several kilograms and can have a somewhat limited sensitivity.
A number of so-called “pre-concentrators” are known that have been used to increase the sensitivity of gas analysis apparatus. The basic principle of a pre-concentrator is to collect molecules to be analysed (analytes) from a flow of gas. After a suitable collection period, the pre-concentrator is reconfigured (e.g. heated) such that the collected analytes are released in a smaller volume for subsequent analysis by an appropriate analyte detector.
An example of a pre-concentrator is described in U.S. Pat. No. 6,171,378 and also by G. Frye-Mason et. al. in the paper ‘Hand-Held Miniature Chemical Analysis System (μChemLab®) for Detection of Trace Concentrations of Gas Phase Analytes’, Micro Total Analysis Systems 2000, 229, 3 (2000). The pre-concentrator of G. Frye-Mason et. al comprises a thin film layer of adsorbent material carried on a substrate and has the benefit of inherently low thermal mass and high thermal isolation. However, a significant drawback of the device is that the essentially planar adsorbent layer provides a low interaction between the fluid and pre-concentrator and hence large adsorbing areas require a large die area.
Another example of a pre-concentrator is described by Wei-Cheng Tian et. al. in the paper entitled ‘Microfabricated Preconcentrator-Focuser for a Microscale Gas Chromatograph’, Journal of Microelectromechanical Systems, Vol. 12, No. 3, June 2003. This pre-concentrator comprises a plurality of channels defined, by deep reactive ion etching (DRIE), in high aspect ratio silicon. Commercially available adsorbent granules (e.g. Carbopack, Carboxen) of an appropriate particle size are located in the channels. Although such a structure provides an increased surface for a given substrate size, the flow of gas still passes over the bed, restricting its contact with the active surface of the adsorbent. Furthermore, the granules simply rest in the channels and are therefore not in intimate thermal contact with the heaters. The device of Wei-Cheng Tian et al thus requires considerable power to heat the adsorbent granules and is quite slow to respond.
A number of gas gating systems are also known for controlling the flow of gas through gas analysis apparatus. In particular, gas flow through a pre-concentrator must be controllably directed to either an exhaust port (e.g. when analytes are being collected by the pre-concentrator) or a detector (e.g. when the pre-concentrator releases adsorbed analytes). Typically, thermopneumatic valves based on diaphragm architectures have been used to provide the required gas flow control function.
Examples of thermopneumatic valves are described in Yang et al, “A MEMS Thermopneumatic Silicon Membrane valve”, Proceedings of IEEE The Tenth Annual International Workshop on Micro Electro Mechanical Systems (MEMS '97), Nagoya, Japan, Jan. 26-30, 1997, pp. 114-118; Grosjean, C. et al “A practical thermopneumatic valve”, Micro Electro Mechanical Systems 1999 (MEMS '99), Twelfth IEEE International Conference, 17-21 Jan. 1999 Page(s): 147-152); and J. S. Fitch et al “Pressure-based mass-flow control using thermopneumatically-actuated microvalves.”, Proceedings, Solid-State Sensor and Actuator Workshop, pp. 162-165 (Transducers Research Foundation, Cleveland, Ohio, 1998).
Disadvantages of prior art thermopneumatic valves include the requirement for active actuation to hold such gas control valves in one of their positions (e.g. power must be continually applied to hold a normally open valve in the closed position). This leads to high power consumption, and thus a high energy budget. Furthermore, the flow of gas through such valves follows a convoluted route and the flow areas are restricted by the fundamental design of the device. Thermopneumatic valves also have a limited response speed and can suffer from hysteresis effects.
After the pre-concentration stage, analytes are released and carried through the gating stage to an appropriate detector. A number of miniature mass spectrometers are known; for example see J. Diaz et al “Sub-miniature double focusing sector field mass spectrometer for in situ volcanic gas monitoring”, Am Soc. of Mass Spectrometry, Sanibel Island, Fla., January 2000 and J. J. Tullstall, et al “Silicon micromachined mass filter for a low power, low cost quadrupole mass spectrometer”, Proceedings of The Eleventh Annual International Workshop on Micro Electro Mechanical Systems, 1998 (MEMS '98), 25-29 Jan. 1998, pp 438-442. Although such miniature mass spectrometers can provide the required analyte analysis, no system is known of truly sub-miniature proportions (e.g. having a volume less than 10 cm3). The generation of suitable vacuum conditions to provide mass spectrometer operation remains a major miniaturisation obstacle.
In a completely unrelated technical field, it is also known that single electron transistor function may be provided by an organic molecule located between source and drain electrodes; see Kubatkin et al, “Single-electron transistor of a single organic molecule with access to several redox states”, Nature, Vol. 425, 16 Oct. 2003. Kubatkin et al describes how the electrical characteristics of an MSET can be used to extract information about the electrical properties of the organic molecule from which the MSET is formed. The MSET described by Kubatkin et al comprises source and drain electrodes that are formed by condensing gold vapour on a substrate held at 4.2 Kelvin. The MSET structure is only operable at low temperature; at room temperature the gold source/drain electrode structures break down.
It is an object of the present invention to provide an improved detector device that mitigates at least some of the above mentioned disadvantages of known systems.